This invention relates generally to air-gas mixing devices of the type used to mix air and gaseous fuel, such as gaseous LPG or natural gas, as a gaseous fuel charge for an internal combustion engine; and more particularly of the type in which air and gas valves for the controlling the air/fuel ratio of the fuel charge are actuated in response to a pressure signal derived from the intake manifold pressure of the engine.
Mixing devices for mixing air with a gaseous fuels are well known. Such a device is commonly attached, together with a butterfly valve, to an intake manifold of an internal combustion engine in lieu of a conventional liquid fuel carburetor when it is desired to operate the engine with gaseous fuel. In operation, the device typically mixes in a mixing chamber air and gaseous fuels in proper proportion for a particular engine load in response to engine intake manifold pressure, which is indicative of engine load requirements, and discharges the mixture in the intake manifold. Air and gas valves within the device control the amount and mixture of the air/fuel charge.
In one such device, shown in U.S. Pat. No. 3,545,948, the air and gas valves are actuated in unison by a diaphragm operated by a pressure signal derived from the intake manifold pressure. The diaphragm opens the valves in response to negative vacuum manifold pressure, and the size and degree of the valve openings, and therefore the air/fuel mixture, at any given manifold pressure are predetermined for the particular fuel and engine on which the device is installed.
A problem with such known air-gas mixing devices is that once installed, the devices cannot feasibly accommodate different gaseous fuels. This is because the fuel valve when opened cannot be independently adjusted to enrich or lean the air/fuel mixture for the particular fuel being utilized. This problem occurs most commonly in the oil refining industry where such devices are installed in industrial engines that are fueled with gaseous fuels left over from the cracking processes. These gaseous fuels vary in their BTU content, depending, for example, on whether the gaseous fuel arises from cracking petroleum for jet fuel or heating oil. Because of the difference in BTU content, it is necessary to adjust the air/fuel mixture for any given engine load when the fuel is changed, which may occur frequently or even continuously. An attempt has been made to accomplish this by providing a mechanical adjustment screw to adjust fuel flow upstream of the fuel inlet of the device, but this is time consuming and the mechanical adjustment screw wears out in time.
Another problem is that such devices do not always provide the most optimum air/fuel mixture for all ranges of engine performance. This is because the air and fuel valves are controlled solely by a fluid pressure signal derived from intake manifold pressure. Although this pressure signal is an indicator of engine load and provides an effective air/fuel mixture for engine performance under most engine conditions, the pressure signal alone cannot be relied upon to provide an optimum air/fuel mixture for all ranges and conditions of engine performance. To accomplish this, it is preferable to adjust the air/fuel mixture from time to time by means of a second control signal from a feedback controller of the type that monitors engine performance in response to inputs from speed, oxygen, vacuum and/or other sensors of engine performance and emits a control signal in response to these sensors. Attempts have been made to adjust the air/fuel mixture by providing an adjustable air by-pass around the fuel valve to lean out a predetermined enriched mixture in response to such a control signal, or by adjusting the pressure of the gaseous fuel to the mixing chamber in response to the control signal. However, these solutions suffer from reliability problems and have inherent time delays which adversely affect engine performance.